A Prony brake generally includes a rotating brake drum or disc connected with the output shaft of a prime mover, such as an internal combustion engine, and stationary friction pads, or brake shoes, that are engageable with the drum or disc in applying a retarding force thereto by frictional contact. The degree of retarding action is dictated by the force with which this frictional contact is applied. A Prony brake as described may be used simply as a power absorption device or, in combination with torque measuring means, as a dynamometer for testing the under-load performance characteristics of the prime mover.
A brake's reactionary force on a prime mover can be expressed or measured in units of weight, be it pounds or kilograms. Reactionary forces are transferred through the brake's internal stationary stator section's shafts. An external extension of the rotary shaft connects to a stationary moment arm or to a fixed torque arm. A torque arm has a defined distance from the centerline of its stationary pivot to a point where the reactionary force is to be measured. This distance can be in inches, feet or in terms of a metric unit. Final measurement of torque is expressed in pounds and feet, i.e., lb/ft, or metric equivalent. Again, a Prony brake only establishes the force or torque reaction to which that force can be measured.
Once a desired torque load has been obtained and torque values have been observed or recorded, it is at that point in time that the operational speed, or rpm (revolutions per minute), must also be observed. With a known torque load (lb/ft) and the operational speed (rpm), output power, or horsepower, can be calculated. Horsepower is the product of torque (lb/ft)×rpm divided by a constant, i.e., 5252. It is at this point, that the device can be classified as a dynamometer.
All inherent reactionary forces both internal and external to the device must be measured accurately. These forces can be created by internal viscus oil shear, friction material drag, or external resistance to rotor travel through a cooling medium. Even though these forces may be small, they must be measured and accumulated in order to obtain maximum reaction force accuracy.
Through the years, Prony brakes have been associated with a brake drum. Brake drums are formed by pouring molten cast iron into a prefabricated sand casting mold. Wooden patterns or aluminum match plates required to make the mold are expensive and require maintenance from time to time. By using this type of casting process, impurities and imperfections are suspended in the casting. Other areas of concern are sand holes, gas holes, and hard spots within the casting. It is the goal that during the machining operations of the brake drum, these impurities and imperfections are removed. Impurities and imperfections at the point of friction can reduce friction material operating lifetime, as well as the life of the brake drum. Brake drums must also be free of porosities which could allow water to penetrate the internal chamber of the brake. Water, oil and friction brakes do not mix very well. Should the brake experience water infiltration, it will go through rapid changes in its coefficient of friction causing the unit to severely vibrate.
Finally, servicing and repair of Prony brakes over the years has been invasive and time-consuming. The performance of periodic, routine servicing of a typical Prony brake can easily extend over several hours, with multiple rotor brakes each requiring individual, independent servicing. For example, fresh maintenance fluid must be directed through each individual brake drum in cleaning or replenishing the maintenance fliud. The sampling of maintenance fluid currently involves a time consuming, invasive procedure, where the contaminated maintenance fluid typically finds its way to a lower brake drum water tank, and becomes mixed with the cooling water. This mixture of coolant water and used maintenance fluid is typically discharged to a drain or to the ground giving rise to pollution issues. The complex, invasive nature of these types of maintenance procedures gives rise to Prony brake low reliability and failures.